Modern film scanners typically employ a linear imaging device such as a charge-coupled-device (CCD) for scanning film images to generate digital signals representative of the image information. A linear charge coupled device (CCD) imager contains a linear array of light detecting photosites which accumulate charge depending on the light energy projected onto them. After some charge accumulation time, the charges in the photosites are transferred to a charge shifting structure so that the charges may be shifted out of the CCD and measured in signal processing circuits to produce a digital data signal for each image pixel, corresponding to photosites in the imaging device, which collectively are representative of the image projected onto the CCD.
Because of manufacturing variability in the CCD, dust or contaminants in the CCD optical path, light source non-uniformity, or other source of variation, the response of the individual photosites may not be uniform. The resulting site-to-site variation in the pixel data may be compensated for by use of gain and offset correction in the signal processing circuits. However, some photosite defects may be bad enough to cause the corresponding pixel data to fall outside the range which may be compensated. If the photosites are small and there are many of them, as in a high resolution scanner, the pixel data which cannot be compensated may not significantly affect the resulting image representation provided that the pixel data from the defective site is replaced with a data value that is consistent with the measurements from adjacent sites. Examples of image scanning apparatus that corrects for bad photosites by substitution of surrounding image pixel data are found in U.S. Pat. Nos. 4,920,428 and 5,168,379. In the '379 patent, a distinctive output signal from a bad photosite is used to indicate the existence of a bad photosite and, when recognized, pixel data from a previous photosite in the scan line or from an adjacent photosite are substituted for the pixel data from the bad photosite. In the '428 patent, photosite correction and attribute data stored in a memory are output in synchronism with photosite processing in the imaging device to apply offset and gain correction on a pixel-by pixel basis. The attribute data identifies a bad photosite and, when encountered, causes a bad pixel correction function to be invoked to substitute pixel data from a previous photosite in the imaging device or an average pixel data value derived from nearby photosites.
It sometimes happens that a bad photosite, or a succession of bad photosites, exists at the beginning of an image scan line. In this instance, there is no data from a previous good photosite in the scan line to substitute for the pixel data from the defective photosite or photosites. It is not acceptable to use pixel data from the last photosite in the previous scan line because it comes from the other end of the image. In the case of a linear imager, there is no vertically adjacent photosite from a previous scan line to serve as a source of good pixel data. Accordingly, there is a need for a fast and simple, low cost solution to providing bad pixel data correction in cases in which a number of one or more successive bad photosites occur at the beginning of an image scan line.